1. Field of the Invention
The present invention relates to a liquid crystal display device suitable for use in electronic notebooks, personal computers, wordprocessors, or the like and more particularly, to a method of forming a topcoat that electrically and physically protects electrodes of such liquid crystal display devices.
2. Description of the Prior Art
Most of the liquid crystal display devices are fabricated, as shown in FIG. 4, by providing a bottom coat (a SiO.sub.2 film) 12, ITO (tin-added indium oxide) electrodes 13, a topcoat 14, and an orientation film (polyimide film) 15 on the inner surface of each of two glass substrates 11, 11, and then filling liquid crystals 17 into the cell gap therebetween. In the figure, designated by reference numerals 10 and 16 are polarizing plates and sealing material, respectively. The bottom coats 12 serve for preventing impurities (e.g. Na) in the glass substrates 11 from moving toward the liquid crystals 17. The topcoats 14, on the other hand, serve for protecting the ITO electrodes 13 electrically and physically when conductive foreign matters of the size equivalent to the above-mentioned cell gap have entered the liquid crystal layer 17, thereby preventing any leak faults.
Conventionally, the topcoats 14 have been formed of a material resulting from dissolving a prepolymer (or monomers), which is composed of Si (or Si and Ti) and either a hydroxyl group (--OH) or alkoxy group (--OR) added thereto, into a high-boiler solvent such as normal methylpyrolidone or dimethylacetamide at a solid ratio of 3 to 10% by weight (hereinafter, the material is referred to as "silica coating ink" or simply as "ink." Such one commercially available is, for example, MOF, Si film or Ti-Si film made by Tokyo Oka Kogyo K.K.). Then this silica coating ink is printed onto the ITO electrodes 13 by a relief-printing technique, and thereafter calcined or baked at a temperature of 250.degree. to 350.degree. C., thus forming the topcoats 14. That is, the silica coating ink is dehydrated and condensed to generate --Si--O--Si-- or --Ti--O--Ti-- (e.g., reaction from silanol to siloxane), the ink thus being oxidized. This imparts insulating property and hardness to the topcoats 14.
It is to be noted that although a temperature of. 500.degree. C. is required to obtain a perfect dehydration and condensation reaction, a baking temperature of 250.degree. C. will do for the topcoats 14 of liquid crystal display devices.
Liquid crystal display devices used especially in electronic notebooks, personal computers, wordprocessors, or the like are required to be lightweight and thin, and also to have a high shock resistance.
To satisfy such requirements, it is desirable to use plastic substrates (e.g., polyether sulfone (PES), polyethylene terephthalate (PET), allyl diglycol carbonyl (ADC), or acrylic resins) instead of the glass substrates 11. In this case, since the plastic substrate does not have a heat resistance against temperatures of more than 200.degree. C., the baking temperature should be limited to 200.degree. C. or lower accordingly. However, if simply the baking temperature is set to 200.degree. C. or lower, the above-mentioned dehydration and condensation reaction will not progress satisfactorily, with the result that the hydroxyl group (--OH), or alkoxy group (--OR) and the solvent remain within the topcoats 14. This makes it impossible to ensure the insulating property and hardness for protecting the electrodes 13, disadvantageously.